(a) Field of the Invention
The present invention relates to a standard photographic lens system and, more particularly, to a standard photographic lens system for which the distance from the first surface of the lens system to the film surface is short, back focal length is long though the focal length thereof is comparatively short, and field angle is wider than those of known standard photographic lens system.
(b) Description of the Prior Art
For known standard photographic lens systems, the field angle (2.omega.) is about 46.degree. to 47.degree.. Among users, however, there is desire to use a lens system with a wider field angle as the standard lens system and, therefore, it is required to develop a lens system for which the field angle is increased to about 56.degree.. When, however, the lens system is used as an interchangeable lens system for a single-lens reflex camera comprising a quick-return mirror, which is most popular among single-lens reflex cameras, it is indispensable that the lens system has a long back focal length. For known Gauss type standard photographic lens systems, the ratio of back focal length to the focal length is 0.8f or less. As a known lens system for which the back focal length is made larger than 0.8f, there is the lens system disclosed in Japanese published unexamined patent application No. 30821/79. The above-mentioned lens system is of such type that the lens elements constituting the cemented doublet to Gauss type lens system arranged in front of stop are separated from each other, and its back focal length is increased to about 0.85f. However, this back focal length is not satisfactorily long.